Our friends over at BigSquidRC.com shared this earlier today:

Closed Cell Tire Inserts have a number of advantages over old school foam. They don’t soak up as much water in wet conditions, they last a lot longer, and they are firmer, resulting in better handling.

Pro-Line is bringing high tech closed cell inserts to the Traxxas 2.8 crowd. These are water safe, reusable, and made to give the 2.8 tires on your truck maximum traction. The set you see in the pictures we have been driving for a couple of weeks now and so far we have been very impressed. We will be putting a lot more miles on them to test durability before our full review goes up in a few weeks.